Challenges and Opportunities in Water Cycle Research:WCRP Contributions

Kevin E. Trenberth • Ghassem R. Asrar

Received: 1 May 2012 / Accepted: 10 November 2012 / Published online: 4 December 2012� Springer Science+Business Media Dordrecht 2012

Abstract The state of knowledge and outstanding challenges and opportunities in global

water cycle observations, research and modeling are briefly reviewed to set the stage for

the reasons behind the new thrusts promoted by the World Climate Research Programme

(WCRP) as Grand Challenges to be addressed on a 5- to 10-year time frame. Those focused

on water are led by the GEWEX (Global Energy and Water Exchanges) project. A number

of GEWEX science questions are being brought forward within GEWEX and the WCRP

under guidance of the Joint Scientific Committee. Here, we describe what are some

imperatives and opportunities for major advancements in observations, understanding,

modeling and product development for water resources and climate that will enable a wide

range of climate services and inform decisions on water resources management and

practices.

Keywords Global water cycle � Hydrological Cycle � WCRP � Precipitation � GEWEX �Water resources � Climate Change � Climate extremes

1 Introduction

Driven mainly by solar heating, water is evaporated from ocean and land surfaces,

transported by winds, and condensed to form clouds and precipitation which falls to land

and oceans. Precipitation over land may be stored temporarily as snow or soil moisture,

while excess rainfall runs off and either forms streams and rivers, which discharge the

freshwater into the oceans, or infiltrates into the soil and percolates to depths to re-charge

the underground aquifers thereby completing the global water cycle (Trenberth et al.

2007a; Fig. 1). Associated with this water cycle, energy, salt within the oceans, and

nutrients and minerals over land are all transported and redistributed within the Earth

K. E. Trenberth (&)National Center for Atmospheric Research, Boulder, CO 80307, USAe-mail: trenbert@ucar.edu

G. R. AsrarWorld Climate Research Programme, Geneva, Switzerland

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climate system. Moreover, water is vital for human existence and is irreplaceable. It is

more than a natural resource that we exploit and often take for granted. Water plays a

crucial role in Earth’s climate, functioning of ecosystems and environment.

Many studies on the global water cycle deal with only specific aspects (see the review

by Trenberth et al. 2007a and other chapters in this monograph). Reliable data on the

surface water budget are often available only over certain regions. Relatively few studies

(e.g., Trenberth et al. 2007a, 2011; Oki and Kanae 2006) have attempted to provide a

synthesized, quantitative view of the global water cycle, and our quantitative knowledge of

the various components, and their variability of the global water cycle is still fairly limited

because of a lack of reliable data for surface evapotranspiration, oceanic precipitation,

terrestrial runoff and several other fields. Regional closure of the water cycle over many

large river basins has been attempted by Vinukollu et al. (2011) and Sahoo et al. (2011)

using satellite data but, unless adjusted, they do not adequately close the water budget, and

the imbalances highlight the outstanding observational and modeling limitations.

Satellite-based observations provide global coverage but may lack continuous coverage

in time and generally require some kind of algorithm to produce a geophysical product that

inevitably has limitations, so the result must be verified against other independent mea-

surements such as in situ observations. However, as the number of analyzed fields grows

with ever increasing satellite data products, it is vital for these to be properly evaluated and

documented for their strengths and weaknesses along with quantifying their uncertainties.

Some of the satellite-based observations limitations may be overcome with in situ

observations because they measure directly the quantity desired, but it is likely a spot

VegetationLandIce

OceanRivers26,350

Lakes 178

Groundwater 15,300

Atmosphere 12.7

Ocean1,335,040

Soil moisture 122

Percolation

40

OceanEvaporation 413

OceanPrecipitation373

LandPrecipitation113

Ground water flow

Surface flow

Hydrological Cycle

Evaporation, transpiration 73

Ocean to landWater vapor transport

* * * * *

* *

Units: Thousand cubic km for storage, and thousand cubic km/yr for exchanges

Permafrost22

40

Fig. 1 The global annual mean Earth’s water cycle for the 1990s. The arrows indicate the schematic flow ofwater substance in various forms. From Trenberth et al. (2007a, b)

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measurement and its representativeness and calibration may not be sufficient to capture the

spatial characteristics, unless a sufficiently large number of such measurements are

obtained. Building and maintaining such large measurement networks have been chal-

lenging, especially over the developing regions of the world where such measurements are

needed most. Blended or hybrid satellite and in situ products are also growing in number

and attempt to capitalize on the strengths of each. Some are produced in a model frame-

work and may involve data assimilation. Nevertheless, with multiple products synthesized

in the framework of the overall water cycle, it is possible to make use of physical con-

straints inherent in a closed water budget, and physical models to help refine all compo-

nents that are not well observed, by taking their uncertainty into account.

2 The WCRP

The World Climate Research Programme (WCRP) mission is to facilitate analysis and

prediction of Earth system variability and change for use in an increasing range of practical

applications of direct relevance, benefit and value to society. The two overarching

objectives of the WCRP are

(1) to determine the predictability of climate and

(2) to determine the effect of human activities on climate.

Progress in understanding climate system variability and change makes it possible to

address its predictability and to use this predictive knowledge in developing adaptation and

mitigation strategies. Such strategies assist the global communities in responding to the

impacts of climate variability and change on major social and economic sectors including

food security, energy and transport, environment, health and water resources (Asrar et al.

2012a). The main foci of WCRP research are

• Observing changes in the components of the Earth system (atmosphere, oceans, land

and cryosphere) and in the interfaces among these components;

• Improving our knowledge and understanding of global and regional climate variability

and change and of the mechanisms responsible for this change;

• Assessing and attributing significant trends in global and regional climates;

• Developing and improving numerical models that are capable of simulating and

assessing the climate system for a wide range of space and time scales;

• Investigating the sensitivity of the climate system to natural and human-induced

forcing and estimating the changes resulting from specific disturbing influences.

The WCRP is sponsored by the World Meteorological Organization (WMO), the

International Council for Science (ICSU) and the Intergovernmental Oceanographic

Commission (IOC) of the United Nations Educational, Scientific and Cultural Organization

(UNESCO).

The World Climate Research Programme (WCRP) is organized as a network of core

and co-sponsored projects, working groups and cross-cutting initiatives. The current core

projects of WCRP are

• Climate and Cryosphere (CliC): The principal goal of CliC is to assess and quantify the

impacts of climatic variability and change on components of the cryosphere and their

consequences for the climate system and to determine the stability of the global

cryosphere.

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• Climate Variability and Predictability (CLIVAR): CLIVAR’s mission is to observe,

simulate and predict the Earth’s climate system with a focus on ocean–atmosphere

interactions in order to better understand climate variability, predictability and change.

• Global Energy and Water EXchanges (GEWEX): GEWEX was previously known as

the Global Energy and Water cycle Experiment but has recently been renamed although

with the same acronym. It focuses on the atmospheric, terrestrial, radiative,

hydrological and coupled processes and interactions that determine the global and

regional hydrological cycle, radiation and energy transitions and their involvement in

global changes such as increases in greenhouse gases.

• Stratospheric Processes And their Role in Climate (SPARC): SPARC has as its

principal focus research on the significant role played by stratospheric processes in the

Earth’s climate, with a particular emphasis on the interaction between chemistry and

climate.

There are also several working groups or councils on modeling and data that coordinate

climate observations, modeling and prediction activities across the entire WCRP. The

coordination of research among the physical, biogeochemical, socio-economic dimension

of global change research has been achieved through Earth System Science Partnership

(ESSP) which is being succeeded by a new initiative entitled ‘‘Future Earth: research for

global sustainability’’ http://www.icsu.org/future-earth.

The Joint Scientific Committee of the WCRP is considering several scientific Grand

Challenges that emerged from the consultation with the global scientific community at a

recent WCRP Open Science Conference to be the major foci for the WCRP activities

during the next decade (Asrar et al. 2012b). They include:

• Provision of skillful future climate information on regional scales

• Regional Sea-Level Rise

• Cryosphere response to climate change

• Improved understanding of the interactions of clouds, aerosols, precipitation, and

radiation and their contributions to climate sensitivity

• Past and future changes in water availability

• Science underpinning the prediction and attribution of extreme events.

Although global water cycle is affected by and affects all of these, we focus only on the

last two challenges that involve water and the hydrological cycle for this monograph.

3 The Global Water Budget and Hydrological Cycle

As the climate changes partly from human activities, the water cycle is also changing

(Trenberth 2011). Moreover, demand for water continues to increase owing to growing

population, enhanced agricultural and industrial development, and other human activities

such as transformation of landscape and construction of dams and reservoirs, so that very

little of the land surface remains in a natural state. This affects the disposition of water

when it hits the ground: how much runs off, and how much finds its way to rivers or

infiltrates into the soil and percolates to depths to replenish the underground water

reservoirs.

The adverse impact of such activities is not confined to quantity and distribution of

water, but also increasingly affects water quality. Water is used in various ways: such as

through irrigation or by consumption in other human activities; reservoirs and artificial

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lakes are used to store water, while dams and other structures are used to control water

flows in rivers. Water is heated and cooled and, as a strong solvent, it is polluted in many

areas.

Many physical scientists have tended to ignore the latter aspects and deal mainly with

the climate system either in its ‘‘natural’’ state or as changed by human activities by mainly

accounting for increased greenhouse gases and changing atmospheric particulates (IPCC

2007). Even in this somewhat simplified framework, it has been challenging to simulate the

hydrological cycle. For example, global reanalyses of most existing observations have

substantial shortcomings in representing the hydrological cycle (Trenberth et al. 2011).

Such shortcomings arise because, while observations are assimilated to ensure a realistic

representation of atmosphere and some Earth surface processes, the analysis increment

ensures that water is not conserved and sources of moisture for precipitation may come

from the increment and not evapotranspiration. Models generally have a lifetime of water

in the atmosphere that is too short, and this affects their ability to transport water vapor

onto land while they tend to recycle moisture locally more than observed.

The main impacts of a warmer climate on global water cycle include the following:

• With warming, higher atmospheric temperatures increase the water holding capacity of

the atmosphere by about 7 % per degree Celsius (e.g., Trenberth et al. 2003).

• Over the ocean where there is ample water supply, the relative humidity remains about

the same and hence the observed moisture goes up at about this rate: an increase in total

column water vapor of about 4 % since the 1970s (Trenberth et al. 2007b).

• Over land the response depends on the moisture supply.

• With more heat in the Earth system, the evaporation is enhanced resulting in more

precipitation. The rate of increase is estimated to be about 2 % per degree Celsius

warming (Trenberth 2011).

• Locally this means increased potential evapotranspiration, and in dry areas this means

drying and more intense and longer lasting droughts.

• Larger warming over land versus the ocean further changes monsoons.

• Precipitation occurs mainly from convergence of atmospheric moisture into the

weather system producing the precipitation, and hence increased water vapor leads to

more intense rains and snow, and potentially to more intense storms.

• More precipitation occurs as rain rather than snow.

• However, higher temperatures in winter over continents favor higher snowfalls.

• Snow pack melts quicker and sooner, leading to less snow pack in the spring.

• These conditions lead to earlier runoff and changes in peak streamflow. Hence, there is

a risk of more extremes, such as floods and droughts.

The pattern of observed changes, so far, indicates wetter conditions in higher latitudes

across Eurasia, east of the Rockies in North America, and in Argentina, but drier conditions

across much of the tropics and subtropics (IPCC 2007; Dai et al. 2009, 2011; Trenberth

2011), and this pattern is referred to as ‘‘The rich get rich and the poor get poorer’’

syndrome (the wet areas get wetter while the arid areas get drier). This pattern is projected

to continue into the future (IPCC 2007), including an increase in probability of the water-

related extremes (IPCC 2012).

Over land there is a strong negative correlation between precipitation and temperature

throughout the tropics and over continents in summer, but a positive correlation in the

extratropics in winter (Trenberth and Shea 2005). The latter arises from the baroclinic

storms that advect warm moist air ahead of and into the storm, combined with the ability of

warmer air to hold much more moisture. The former arises from the nature of the

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atmospheric circulation interactions with land. In cyclonic conditions, increased cloud and

rain provide more soil moisture and thus partitions the decreased surface energy more into

latent energy (higher evaporation) instead of sensible heat (lower temperatures). Anticy-

clonic conditions favor sunshine (more available energy), less rain and soil moisture, and

the larger surface energy raises temperatures instead of evaporating moisture. The result is

more likely either hot and dry or cool and wet conditions, but not the other options.

On global land, there is large variability in precipitation from year to year and decade to

decade associated especially with the El Nino-Southern Oscillation (ENSO) but there has

been an increase overall in land precipitation (Fig. 2). The two wettest years are 2010 and

2011. In particular, major flooding in Pakistan, Australia, and Colombia was associated

with record high sea surface temperatures (SSTs) in the second half of 2010 into 2011

(Trenberth 2012) and led to a dramatic drop in sea level of about 5 mm (Fig. 3). The

prospects for more intense precipitation but longer dry spells lead to the increased risk of

flooding and drought, which pose major challenges for the society at large and those who

have to manage water resources for food, fiber and energy production, and human con-

sumption and leisure. We therefore view observing, understanding, modeling and pre-

dicting the global water cycle as a grand science challenge.

4 Grand Challenges

A Grand Challenge should inspire the community to want to be involved; it needs to be

specific and focused while identifying barriers and ways to advance the science, and it

must capture the imaginations of funding agencies, science program managers and the

public. It should also provide a vehicle to encourage the different WCRP panels to interact

in pursuing a common goal. It must provide a way forward that is tractable, perhaps via

new observations (e.g., from satellites), computer and model advancements, and ideas. It

must matter, as shown by answers to questions on possible benefits to society by providing

Fig. 2 Annual mean anomalies in global land precipitation from 1900 to 2011 in mm; from NOAA.http://www.ncdc.noaa.gov/sotc/global/2011/13

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the science-based information to address impacts of climate variability and change to food,

water, health, energy, biodiversity and so on.

The GEWEX Science Steering Group (SSG) has identified four GEWEX Science

Questions (GSQs). These emerged from in depth discussions and subsequent circulation to

all GEWEX Panel members for commentary. They were then posted on the GEWEX web

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site for open commentary. They have also been presented to the WCRP Joint Scientific

Committee for comment, and the outcome is what we present here. Three of these GSQs

deal with water and two of them are combined into a more general water resource Grand

Challenge for WCRP that also encompass scientific activities coordinated by the CliC,

CLIVAR and SPARC projects. This is outlined in Sect. 4.1 along with a number of more

specific questions. The third GSQ is part of a WCRP-wide theme of extremes, and those

extremes related to water are discussed in Sect. 4.2. The other GSQ relates to energy and

processes.

4.1 Grand Challenge on Water Resources

How can we better understand and predict precipitation variability and changes, and how

do changes in land surface and hydrology influence past and future changes in water

availability and security?

These questions focus on the exploitation of improved data sets of precipitation, soil

moisture, evapotranspiration, and related variables such as water storage and sea surface

salinity expected in the next 5 to 10 years. These will allow us to help close the water

budget over land and provide improved information for products related to water avail-

ability and quality for decision makers and for initializing climate predictions from seasons

to years in advance. The improvements will come from ongoing and planned satellite

missions (see below) as well as greater use of in situ observations; their evaluation and

analysis to document means, variability, patterns, extremes and probability density func-

tions; their use to confront models in new ways and to improve our understanding of

atmospheric and land surface processes that in turn improve simulations of precipitation;

and new techniques of data assimilation and forecasts that can lead to improved predictions

of the hydrological cycle across scales, from catchments to regions to the entire globe,

including hydrogeological aspects of ground water recharge. In particular, attention is

needed on the use of realistic land surface complexity with all anthropogenic effects taken

into account, instead of a fictitious natural environment. This encompasses all aspects of

global change, including water management, land-use and land-cover change, and

urbanization. The ecosystem response to climate variability and responsive vegetation to

such changes must be included, as must cryospheric changes such as dynamics of per-

mafrost, thawing and changes in mountain glaciers. The focus on these scientific questions

should lead to improved understanding and prediction of precipitation and water vari-

ability, enhance the evaluation of the vulnerability of water systems, especially to

extremes, which are vital for considerations of water security and can be used to increase

resilience through good management and governance.

The 21st century poses formidable challenges for the sustainable management of water

resources at all levels, from the local, regional to the global scale. Water is a basic

requirement for life, and effective water management is needed to provide some of soci-

ety’s most basic needs. However, demand for water resources is increasing, due to pop-

ulation growth and economic development, while water resources are under pressure

globally from over-abstraction and pollution. This is increasingly leading to competition

for water, at local, regional and international levels. Environmental change is adding

additional pressures. Consequently, there are growing issues of vulnerability and acces-

sibility to water, both of which are highly relevant for society. Anthropogenic influences

are changing land and water systems, redefining the state of drainage basins and the rivers

and groundwater aquifers that supply the bulk of renewable freshwater to society. Wide-

spread land-use changes, associated with population increases, urbanization, agricultural

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intensification and industrialization are changing hydrological systems in complex ways,

and on many of the world’s major rivers, water management is changing flows, often with

severe effects on downstream users, aquatic ecosystems and freshwater discharges to the

world’s seas and oceans. Superposed on these pressures, expected climate change and

climate variability can combine to create extreme and perhaps unprecedented conditions

which have high impact consequences for human populations, economic assets and critical

physical infrastructure. This unique combination of pressures has exposed weaknesses in

current water governance and management. It has increased the awareness of uncertainties,

the complexity of the systems to be managed, and the need for profound changes in policy

and management paradigms, as well as governance systems.

The World Climate Research Programme (WCRP) has a unique role to play in

developing the new scientific understanding and modeling and prediction tools needed for

a new era of global water management. WCRP mainly through GEWEX, and based on

significant contributions from CLIVAR and CliC projects, is well poised to motivate a new

generation of land surface and global hydrological models, building on recent develop-

ments in Earth observations, that represent the dynamics of major managed water systems.

The modeling activities have an equally important role in motivating a new generation of

weather-resolving climate models that are capable of simulating and potentially predicting

the basic modes of variability, whether arising from sea surface temperature and ocean,

land surface moisture, sea ice or other sources that are known to drive global precipitation

variability and extremes on seasonal to decadal time scales. Such prediction systems are

increasingly necessary to address regional impacts of climate change.

The vast majority of water comes from precipitation—either directly or indirectly

through runoff from distant locations. From a climate perspective, it is therefore an

imperative to understand the natural variability of precipitation in the system, as well as its

susceptibility to change from external forcings. Within GEWEX, the Global Precipitation

Climatology Project (GPCP) (Huffman et al. 2009) has been a focus of improving esti-

mates of precipitation. Because of its inherently intermittent nature, it is a major challenge

to determine precipitation amounts reliably with a few instantaneous observations of rates

such as from available satellites. Improved observations and analysis products related to

precipitation and the entire hydrological cycle and their use in evaluating and improving

weather, climate and hydrological models are important and tractable over the next 5 to

10 years.

The specific questions that will be addressed over the next 5–10 years include:

• How well can precipitation be described by various observing systems, and what basic

measurement deficiencies and model assumptions determine the uncertainty estimates at

various space and time scales? Despite the significant improvements in many observing

systems during the past two decades, the uncertainty in precipitation estimates lies not

only in the measurement error itself, but in the space/time interpolation of a naturally

discontinuous and intermittent field and/or in the assumptions needed to convert a

physical measurement from remote sensing into a precipitation amount. Critical water

source regions often reside in complex terrain where sampling issues, remote sensing

artifacts and limitations are compounded. The errors are not static but instead depend on

the nature of the precipitation itself. Focusing on the large-scale environment

responsible for the precipitation therefore holds hope to build not only better rainfall

products, but characterizing the uncertainties in a verifiable manner as well. Regional

hydroclimate projects provide detailed understanding that translate the large-scale

information into usable information for decision makers.

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• How do changes in climate affect the characteristics —(distribution, amount, intensity,

frequency, duration, type) of precipitation with particular emphasis on extremes of

droughts and floods? Increased temperatures and associated increases in lower

tropospheric water vapor, by making more water vapor available to storms, will very

likely increase the intensity of rains and snows, increasing risk of severe floods.

Changes in seasonality, shifts in monsoons, changes in snow-melt and runoff, and so on

are also part of this question which is elaborated on in the ‘‘extremes’’ science question.

• How do models become better and how much confidence do we have in global and

regional climate predictions of precipitation? A challenge to the Earth System science

community is to develop improved global models. Scientists are beginning to run

global climate models at sub-10-km resolution, resolving meso-scale weather including

the most extreme tropical storms. These need to be coupled to the ocean and land and

will require a new generation of parameterizations that better reflect what processes are

and are not resolved in such models. These models can potentially revolutionize our

ability to correct long-standing model biases, minimize the need for downscaling and

provide predictions of regional impacts and changes in extremes from months to

decades ahead. There is great need to quantify the uncertainty in precipitation

projections and predictions, especially at regional scales. Starting with improved

uncertainties in the climate observations of precipitation, new and improved

diagnostics must be developed to test the robustness of model predictions in different

regimes. Knowing the uncertainties is critical if predictions of the mean precipitation

and its distribution are to be used in local planning efforts.

• How do changes in the land surface and hydrology influence past and future changes in

water availability and security? While the land surface has small heat capacity and heat

moves slowly via conduction, the water flow and storage vary enormously. Land has a

wide variety of features, topography, vegetation cover and soil types and consists of a

mixture of natural and managed systems. Land plays a vital role in carbon and water

cycles, and ecosystems functions and services. Of particular need of attention is use of

realistic land surface complexity in hydrological models with all anthropogenic effects

included instead of a fictitious natural environment. This includes all aspects of global

change including water management, land-use and land-cover change and urbanization,

and their feedbacks to the climate system. There is a need to address terrestrial water

storage changes and close the water budget over land through exploitation of new

datasets, data assimilation, improved physical and biogeochemical understanding and

modeling skill across scales, from catchments to regional to global with links to the

entire hydrological cycle.

• How do changes in climate affect terrestrial ecosystems, hydrological processes, water

resources and water quality, especially water temperature? The ecosystem response to

climate variability and responsive vegetation must be included but is mostly neglected

in today’s climate models. Cryospheric changes such as permafrost thawing, changes in

the extent, duration and depth of seasonal snowpacks, and changes in mountain glaciers

must also be included. How changes in vegetation affect the hydrological cycle and

climate in turn are vital. Feedbacks, tipping points and extremes are of particular

concern to all economic sectors and regions, globally. The scientific knowledge of

water cycle should enhance the evaluation of the vulnerability of water systems,

especially to extremes, which is vital for considerations of water and food security and

can be used to increase their resilience through good management practices and

governance.

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• How can new observations lead to improvements in water management? Over the last

few decades, in situ observations of land surface hydrological variables, such as

streamflow, rainfall and snow have generally been in decline. Regional estimation of

evapotranspiration remains a significant challenge. At the same time, new observation

methods, such as weather radars, flux towers and satellite sensors have led to different

types of measurements, and challenges for their incorporation in the hydrological

models used for hydrological prediction and water management. One example is soil

moisture, which in most models essentially acts as a buffer between the land forcings

(mostly precipitation and evapotranspiration) and runoff, and whose characteristics are

defined by the internal model parameterizations that control runoff production.

Sustained measurements of soil moisture are critically important to understanding,

modeling and prediction of the water cycle.

• How can better climate models contribute to improvements in water management?

Regional precipitation predictions remain a challenge at all timescales from seasonal

forecasting out to centennial climate change. However, there are limited regions with

forecast skill on seasonal timescales, associated mainly with ENSO, and broad scale,

zonally averaged precipitation changes associated with climate change appear to be

detectable. The challenge now is to maximize the skill and reliability of predictions of

regional rainfall changes on all timescales, for all regions around the world. This

requires better understanding and model simulation of the tele-connections and drivers

of regional climate such as changes in the oceans and cryosphere that are relevant to

regional precipitation. Subsequent improved climate prediction systems and better

dissemination of climate prediction information must be developed to deliver the

envisioned information and their ultimate benefit to society.

Prospects for advancements are excellent on this Grand Challenge because of new

observations already underway and those planned for the ensuing decades and the growing

interest in climate predictions on all timescales. Key areas of development include

1. A new Global Precipitation Mission as detailed at http://pmm.nasa.gov/GPM.

‘‘Through improved measurements of precipitation globally, the GPM mission will

help to advance our understanding of Earth’s water and energy cycle, improve

forecasting of extreme events that cause natural hazards and disasters, and extend

current capabilities in using accurate and timely information of precipitation to

directly benefit society.’’ The joint US National Aeronautics and Space Adminis-

tration (NASA)/Japan Aerospace Exploration Agency (JAXA) mission’s Core

Observatory is scheduled for launch in 2014. Most of the world’s major space

agencies will participate in this mission through the contribution of constellation

satellites to obtain the desired revisit times to roughly 3 h, over the entire Earth.

2. Closely related missions such CloudSat (a NASA mission with components from the

Canadian Space Agency to measure clouds and light precipitation) and EarthCARE,

a European Space Agency (ESA) mission (http://www.esa.int/esaLP/SEM75

KTWLUG_LPearthcare_0.html) to advance our understanding of the role that

clouds and aerosols play in the climate system), due for launch late 2015, that will

make important contributions to the global precipitation estimates.

3. New satellite sensors such as soil moisture and ocean salinity (SMOS) (an ESA

mission to map soil moisture and sea surface salinity), Aquarius (a NASA/Space

Agency of Argentina mission to improve sea surface salinity) and future soil

moisture active passive (SMAP) data (a NASA mission dedicated to measuring soil

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moisture and the freeze/thaw cycle), produce or will produce estimates of near-

surface soil moisture that can be used to diagnose or update model estimates, and

Gravity Recovery and Climate Experiment (GRACE) (a joint NASA/German

Aerospace Center (DLR) mission to map gravity anomalies and thus detect changes

in water storage), now provides a nearly decade-long record of total water storage,

albeit at coarse spatial resolutions. The GRACE follow on mission is intended to

enhance the spatial resolution of such measurements and provide continuity of

measurements over the future decade. The planned surface water and ocean

topography (SWOT) mission will provide observations of lake and reservoir surface

area and levels, from which changes in storage of over 7,000 km3 of the estimated

8,000 km3 of reservoir storage globally will be available at 1–2-week intervals. In

addition, in situ observations from buoys to Argo floats will help close the water and

energy budgets over the oceans.

4. A dedicated snow hydrology mission such as ESA’s Cold Regions Hydrology High-

Resolution Observatory (CoReH2O) will enable better understanding of the role

snow hydrology plays in the regional/global water cycle, especially in mountainous

regions of the globe that depend mainly on snow as a source of fresh water for human

consumption, food production and industrial activities (e.g., California, Tibetan

Plateau, La Plata Basin, etc.).

5. Improvements in communication and data exchange policies help create higher

resolution global surface maps of precipitation and soil moisture based upon both

local very dense networks of high-resolution measurements as well as surface radar

networks where these are available. Significant gains are expected from high-

resolution gridded products being developed by GEWEX and other projects based on

in situ data as well as inventories of long-term in situ precipitation time series

focused on engagement of these data into validation, error estimation and

intercomparison efforts. The use of improved error statistics to develop new

blending algorithms and fusion techniques capable of bringing together precipitation

measurements with distinct error characteristics (e.g., gauges, radar, satellites and

models) into a consistent physical framework. Advances in data assimilation

techniques allow more precipitation information to be incorporated into Numerical

Weather Prediction models.

6. Estimates of fluxes of moisture from surface are improving through the use of flux

tower and other observations over land, feeding into improved estimates of

evapotranspiration as part of the GEWEX Landflux and ocean flux projects.

7. The production of an Integrated Water and Energy product by the GEWEX data and

assessments panel (GDAP) can be used to explore linkages between hydrology and

energy variables in the Earth System which in turn provides a much improved basis for

evaluating models on all aspects of the water cycle. Advanced diagnostic methods that

use the observed variables and their co-variability are used to diagnose not only problems

in the model output, but also assess model processes and potential improvements to these

processes in order to better represent the observed climate behavior.

8. Incorporate more realistic land surface hydrology into land surface models, including

water management, land management and land-use and land-cover change, as well as

improved process representation (including cryospheric processes). The envisioned

new information is expected to be revolutionary in terms of the management of trans-

boundary rivers, but current climate models have no mechanisms for use of this

information, since most do not represent the effects of water management.

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9. New methods must be developed to address water system vulnerability, particularly

to extremes. Quantification of the uncertainty in each of the elements of the global

water-balance, including the managed aspects, in a consistent manner is required.

Further, there is a need to communicate uncertainties, manage expectations, address

the needs of water management under uncertainty (e.g., building resilience).

10. Several other developments in modeling are progressing and advances appear likely.

These include development of improved precipitation downscaling methods,

particularly for mountainous and arid regions; evaluation of the hydrological

dynamics of land surface models with newly available data; prediction of stream

temperature as a diagnostic tool in land surface models; improving freshwater fluxes

to the world’s seas and oceans; and including the known climate feedbacks in off-line

land surface change assessments. Water demand models and assessments to land

surface and hydrological models must be linked at the global scale.

11. Demonstration of the usefulness of GEWEX, and Global Climate Observing System

(GCOS) and WCRP coordinated data products is required along with new tools and

provision of derived information for water resources management. The new tools

include cross-scale modeling, ensemble hydrological prediction, data assimilation,

and data analysis and visualization.

There are multiple benefits, and the results are critically important for society. In

addition to greatly improved knowledge about land water resources and ocean salinity, and

the causes of their variations, much improved models will allow better predictions of the

variability and change on all time scales from seasonal to centennial and from global to

continental to basin scales. Predictions with quantified uncertainties provide invaluable

information for water managers and users, including decision makers at many levels

associated with food and water security. These developments would naturally serve to push

WCRP research and development priorities, as users provide feedback on weaknesses and

further needs for information.

The information provided also feeds into the development of a ‘‘Global Drought

Information System’’. Such a system would provide a user anywhere in the world access to

information on our current understanding of drought in that region (e.g., role of ENSO,

Pacific Decadal Oscillation, global warming, etc.), the history of drought in that region

(with access to various data, time series, indices, etc.), current conditions (monitoring

results), the results of near real-time attribution (our understanding of the current condi-

tions) and regularly updated forecasts from months to years in advance (with consistent

estimates of uncertainties).

The system would naturally build on the various investments being made in observa-

tions (including reanalysis), drought research, and modeling and forecasting capabilities

(e.g., the various national and international multi-model ensemble (MME) efforts such as

the WMO lead center for long range forecasts: http://www.wmolc.org). The system would

be built hand-in-hand with the user community and would have to be sustainable and

refreshable as new datasets, better understanding and better modeling capabilities become

available. It would naturally serve to push WCRP research and development priorities, as

users provide feedback on weaknesses and further needs (analogous to how the weather

community is continuously being pushed for better weather forecasts). These are the

envisioned products and information to be provided by the network of organizations and

centers through Global Framework for Climate Services (GFCS) and Future Earth (WMO

2011; Asrar et al. 2012a).

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4.2 Grand Challenge on Water Extremes

How does a warming world affect climate extremes, especially droughts, floods and heat

waves, and how do land area processes, in particular, contribute?

A warming world is expected to alter the occurrence and magnitude of extremes such as

droughts, heavy rainfalls and floods, as well as the geographic distribution of rain and snow.

Such changes are related to an acceleration of the hydrological cycle and circulation changes

and include the direct impact of warmer conditions on atmospheric water vapor amounts,

rainfall intensity and snow-to-rain occurrence. How well are models able to handle extremes

and how can we improve their capability? New improved and updated data sets at high

frequency (e.g., hourly) are needed to properly characterize many of these facets of Earth’s

climate and to allow for assessment against comparable model data sets. New research

activities are needed to promote analyses quantifying which changes are consistent with our

expectations and how we can best contribute to improving their prediction in a future climate.

Confronting models with new observationally based products will lead to new metrics of

performance and highlight shortcomings and developmental needs that will focus field

experiments, process studies, numerical experimentation and model development. New

applications should be developed for improved tracking and warning systems, and assessing

changes in risk of drought, floods, river flow, storms, coastal sea-level surges and ocean waves.

There is major concern that the occurrence, character and intensity of extremes will

change in the future as the climate changes due to human activities, and this will have

enormous consequences for society and the environment. Yet addressing changing

extremes satisfactorily is a daunting task, and it will be difficult to keep up with society’s

expectations. As noted above, huge improvements in near-global spatial and temporal

coverage for precipitation, soil moisture and other hydrological variables provide oppor-

tunities for new datasets, products, improved models and model applications, making it an

opportune time to fully address extremes.

The climate system does not neatly package such extremes. Extremes may be highly

localized in time and in space. Drought in one region frequently means heavy precipitation

not that far away. The worst extremes are generally compound events which often are

consequences of a chain-of-events that may be related at the global scale despite their

regional implications. Flooding may be accentuated due to saturated soils from previous

storms and/or from snowmelt. Furthermore, coastal flooding may involve storm surge

effects, local precipitation and remote snowmelt signals.

Because of its importance, there are many efforts focusing at least in part on extremes

within WCRP. One focus is on drought, although there is certainly interest in other hydro-

meteorological extremes and related issues, such as statistical analyses. WCRP, mainly

through CLIVAR, also addresses tropical and extratropical cyclones and associated marine

storms as well as extreme sea-level variability and change that is connected to storm surges.

GEWEX with its focus on the water cycle and on land surface processes with strong obser-

vational capabilities from global to local and with numerous links with society is a natural

‘home’ for addressing many types of extremes. The question is what is missing and what can

be done within GEWEX to move ahead? The main GEWEX focal point is to increase efforts

on hydrometeorological extremes including drought, heat waves, cold outbreaks, floods,

storms and heavy precipitation events including hazardous winter snowfalls and hail.

The specific questions that will be addressed over the next 5–10 years include

• What are the short-term, mid-term and strategic requirements for the existing

observing systems and datasets, and which observations are needed to accurately

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quantify trends in the intensity and frequency of extremes on different space/time

scales? Despite a continuous improvement in most observing systems, high-frequency

information (e.g., hourly precipitation) required to properly assess extremes is often not

made available and shared. New satellite observations and the synthesis of all

observations will help and may free up some data. Metrics for quantifying extremes

need to be assessed, and new ones should be introduced to improve diagnostics of

extremes and scale them to different areas. It is necessary to determine for which

regions (national observing systems) the requirements are close to being satisfied and

where they are not. There is an urgent need for research on design, development and

maintaining optimum observing systems, the regular analysis of their adequacy/

inadequacy for future investments in such systems.

• How can models be improved in their simulation and predictions or projections of the

magnitude and frequency of extremes? Current models have difficulty in simulating the

hydrological cycle, and they typically have problems handling the diurnal cycle. Model

resolution is insufficient in most cases to simulate many of the extremes of interest,

including floods with scales of a few kilometers and even drought whose worst-affected

areas are typically in areas only of order a few 100 km or less. Model parameterizations

addressing precipitation, convection and clouds are insufficient for accurate simulation

and timing of many extreme events. Models need to be confronted with the new

observational products in innovative analyses and with new diagnostics and metrics of

performance. This includes numerical weather prediction and climate models. There

are conceptual difficulties in validating model results against observations, first of all

associated with (but not limited to) co-location in space and grid cell data versus point

measurements. Many observational products are developed independent of models so

that gridding projections and associated error characteristics are often different from

model-derived data products thus making their direct inter-comparisons very difficult if

not impossible. Focused investments by space agencies (e.g., ESA and NASA) to make

the observational products consistent and inter-comparable is quite timely. Such efforts

facilitate research on observations and make inter-comparisons with models much

easier and enhance the use of observations by the modeling community.

• How can the phenomena responsible for extremes be better simulated in models? Many

phenomena that are responsible for extremes are not well simulated in models; some

because of resolution (such as tropical storms and highly localized precipitation

events), but also others that are resolved (such as blocking anticyclones). As well as

statistical analyses, studies should examine the phenomena responsible for extremes,

whether and how well they are depicted in models, and how to overcome incompatible

resolution requirements. Developmental needs should be used to focus field programs,

process studies and numerical experimentation.

• How can we promote development of applications for improved tracking and warning

systems arising from extremes? It is essential to develop ways to better assess changes

in risk of drought, floods, river flow, storms, coastal sea-level surges and ocean waves.

Such information has the greatest benefit to society for management of risks associated

with these events to reduce their adverse impacts. In most cases, such applications will

be done in conjunction with the CLIVAR and CliC projects and made available through

networks sponsored by GFCS and other regional climate information systems.

Prospects for advancements are excellent on this question because of new observations,

research, modeling and prediction activities already underway and planned. A number of

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specific, short and near-term activities are envisioned that will enable progress on this

Grand Challenge. Key areas of development include

1. Utilization of the new global and regional datasets outlined above and from improved

data assessment (within the GEWEX Data and Assessments Panel) to better characterize

extremes on different spatial scales and, with the WCRP Modeling Council, promote

evaluations of model results, potentially with one or more workshops in 2014–15.

2. Ensure strong involvement in the Global Drought Information System. This focuses on

one particular type of extreme but the effort may also act as a prototype for dealing

with all types of extremes in the future. In particular, GEWEX and CLIVAR will

develop trackable actions on monitoring and quantification of the global distribution of

droughts and their trends using observational information, model development, land

area factors governing drought and societal interactions.

3. Facilitate a number of inter-comparison projects aimed at comparison of character-

istics of extremes in different data sets (in situ, reanalyses and satellites), and revealed

by different models.

4. Initiate a parallel activity centered on capabilities of statistical methodologies to deal

with the complexity of extremes, including their clustering in space and time and with

sparse and regionally unevenly distributed data.

5. Initiate multi-methods activities and encourage documentation and data inventory

centered on a few mega-extreme events (for example, catastrophic flooding, droughts,

unusual storm patterns) to enable further analysis with observations and models,

ensure that all their aspects are comprehensively addressed, and with special attention

on assessing their likelihood in the future. This activity may be facilitated by bringing

teams together and should build in flexibility with adaptable approaches as one learns

by doing. It has the advantage that the results are immediately relevant.

6. Examine cold season extremes such as snowstorms, rain-on-snow episodes, freezing

precipitation and prolonged cold weather events with CliC and other international and

national research programs/projects.

There are multiple benefits, and the results are important for society. Drought has

devastating consequences whenever and wherever it occurs. Water resources can be

strained, and adverse effects occur in agriculture. Heat waves are often but not always

linked with drought. Health effects can be profound. Prolonged cold weather episodes are a

critical feature of mid- and sub-polar latitudes in winter. They are disruptive and costly.

Isolated extreme rainfalls as well as continuous periods of heavy and moderate precipi-

tation occur everywhere with numerous impacts including flooding, devastation of eco-

systems and havoc in urban regions. Storms in different parts of the world are the means by

which precipitation, often linked with strong winds, occurs, and changes in their paths,

intensity and frequency have enormous consequences, sometimes devastating. Warming

conditions imply that regions accustomed to receiving snow should experience more rain,

and changing times of runoff and peak stream flow, with large consequences for ecosys-

tems, hydrological risks and water resources.

These examples highlight the importance of progress in the area of climate extremes,

both in terms of their observations and analysis, and in terms of improved modeling and

prediction. In summary, WCRP through GEWEX, CLIVAR and CliC and its seamless

modeling framework across space and time scales (e.g., Working Groups on Coupled

Modeling and Numerical Experimentation (WGCM and WGNE)) will focus great attention

on extremes, including research on detection and attributions of causes and consequences

of such events over the next 5–10 years. By doing so, it will be carrying out its very natural

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role of addressing the estimation, modeling, understanding and future projection of

extremes with a particular focus over land.

5 Conclusions

The successful implementation of WCRP Grand Challenges and associated science

questions described in this chapter depend significantly on GEWEX Imperatives: obser-

vations and data sets, their analysis, process studies, model development and exploitation,

applications, technology transfer to operationalize results, and research capacity devel-

opment and training of the next generation of scientists. They involve all of the GEWEX

panels and will benefit greatly from strong interactions with other WCRP projects such as

CLIVAR and CliC and other sister global change research programs such as the Inter-

national Geosphere-Biosphere Programme (IGBP), International Human Dimensions

(IHDP), etc.

Closure of the observed regional and global water budget over the past decade has

progressed significantly, but remains a major challenge. Thus it continues to be a science

imperative for the research community to better observe and understand all aspects of the

water cycle in order to improve models that can predict reliably its future variability and

change as a major source of information for decision makers for water resources, food

production and management of risks associated with extreme events. Many potential

products could be invaluable to water resource managers on several time horizons,

extending well beyond the 1-week weather scale to seasonal, inter-annual and decadal

predictions, and climate change projections.

Acknowledgments The research of Trenberth is partially sponsored by NASA under grantNNX09AH89G. We thank Howard Wheater and many other WCRP colleagues for discussions, and espe-cially we thank GEWEX scientists who contributed to the GSQs. The National Center for AtmosphericResearch is sponsored by the National Science Foundation.

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